1. Field of the Invention
The present invention relates to a radiation-sensitive positive resist composition which is sensitive to ultraviolet rays, far ultraviolet rays (excimer laser and so on), electron rays, ion beams and radio active rays such as X-rays.
2. Description of the Related Art
Recently, particularly in the production of integrated circuits, miniaturization has proceeded as the integration level has increased, which results in demands for formation of patterns on the submicron order and more excellent resolving power. According to the conventional processes for the production of integrated circuits, light exposure is accomplished by placing a mask with intimate contact to a substrate such as a silicon wafer. It is said that this process cannot make patterns thinner than 2 .mu.m. Instead of such conventional processes, the reduction projection exposure system attracts attention. According to this new system, a pattern of a master mask (reticle) is projected on the substrate with reduction by a lens system, whereby exposure is accomplished. This system realizes a resolving power of 1 .mu.m.
One of the serious problems in this system is low throughput. Namely, in this system, the total exposure time for exposing a wafer is very long because of divided and repeated light exposure, unlike a batch light exposure system such as the conventional mask contact printing method.
For solving this problem, not only the improvement of the apparatus but also increase in sensitivity of the resist to be used are important. If the exposure time can be shortened by the increase of the sensitivity, the throughput, and in turn the yield, can be improved.
On the other hand, as the distance between the adjacent two lines is decreased with the increase of the integration level, dry etching is predominantly used rather than wet etching. Then, the photoresist should have better heat resistance than ever.
When the positive photoresist now in practical use is viewed from this standpoint, its sensitivity, resolving power and heat resistance are not necessarily satisfactory. Generally, the positive photoresist has lower sensitivity than the negative photoresist, and improvement in the sensitivity of the former is desired.
For increasing the sensitivity, it is easiest to decrease a molecular weight of the novolak resin used in the positive photoresist. The decrease of the novolak resin molecular weight accelerates dissolution of the photoresist in an alkaline developing solution so that the apparent sensitivity of the photoresist is increased.
This method, however, has a very serious disadvantage such as deterioration of the heat resistance of the photoresist. Moreover, it encounters some problems such as large film thickness loss in an unexposed area (reduction of so-called film thickness retention), deterioration of the shape of the pattern, and deterioration of the .gamma.-value because of the small difference of the dissolving rates in the developing solution between the exposed area and the unexposed area.
In view of this, positive resists possessing sensitivity, resolving power and heat resistance at the same time have not been on the market up to now. Trying to improve one of these three characteristics, at least one of the remaining characteristics tends to be impaired.